Integrated Crop Management News

Although there are many ways weeds escape control in crop fields, one of the leading causes of waterhemp control failures is emergence of plants following postemergence herbicide (POST) treatments. Waterhemp requires more than twice as many growing degree days to reach 50% emergence as giant foxtail or velvetleaf (Figure 1), resulting in much of the population emerging after mid-June.  

Plots were established at two locations in 2018 (ISU Northwest Research Farm and ISU Northeast Research Farm). Syngenta NK S24-K2 brand soybean was used at both locations. Seven insecticidal groups/subgroups were evaluated in 2018. Seed did not have a pesticidal seed treatment unless specifically stated. At both locations, soybean aphid peaked in August (four perplant at Northeast Research Farm and 22 perplant at Northwest Research Farm).

With this year’s harvest of soybeans delayed beyond what is considered an ideal window of time, the opportunity for diseases to infect seed pods and in some instances, to the seed itself, was greatly increased. Across the state and the north central region, seed suppliers have reported that this year’s crops of seed soybean are frequently testing positive for the Diaporthe fungus (Phomopsis seed decay), which is resulting in lower than normal germination rates of seed.

As the season approaches its conclusion and harvest conditions are most challenging, there are few things worth remembering to protect and sustain soil health. At this time, soil is susceptible to compaction due to rain and saturated soil conditions. Soils remain saturated longer at this time of the year since water use by crops is negligible and there is low water evaporation due to cool temperatures.

Late harvest and the rush to get grains out of the fields may present an opportunity to rethink the need for tilling fields this fall or not. The question to ask is, “Do I need to till this fall?” Given the economic and environmental challenges farmers are facing, the answer in most cases is no. With harvest under way, now is a good time to start thinking about this decision.

As of October 14, 2018, Iowa soybean harvest was only about 20% complete, making it the latest soybean harvest on record. This was caused by the prolonged heavy rains in September and early October. As a result, field losses, abnormally high harvest moisture content and moldy/weathered soybeans are all issues this year.

Above normal rains in September have slowed field crop dry-down. Coupled with early season drought in South Central and Southeast Iowa and above-average rainfall in the Northwest, there is high risk of reduced grain quality. Corn and soybeans remaining in the field are currently exposed to excessive moisture that encourages the growth of ear molds. Moldy kernels are counted in total damage, thereby affecting the overall grade of the corn.

Rain events during September and October have created challenging conditions not only for timely harvest of corn and soybean crops but also for the impact harvest will have on the soil. These wet conditions coupled with a drop in air temperature will slow harvest operations. Soils are too wet for traffic from heavy equipment, making them susceptible to compaction during harvest operations.

This year continues the chain of years with unusual harvest conditions driven by rapid weather changes in the latter part of the growing season. In mid August, crops were significantly ahead of schedule in terms of maturity. Heat and moisture in May and June accelerated the pace of development, to the point that signs of maturity were evident by the 15^th of August.

There have been flooded fields with water over the grain in Southwest, Northwest and East Central Iowa this year. This was caused by intense rains over Labor Day and the following weekends. Poor stalk strength causing downed corn has increased the amount of grain covered by flood waters.

Harvest is just around the corner for many Iowa farmers and now is a good time to consider options to reduce movement of weed seed between fields with harvest equipment. While we may not think of it during harvest time, combines are extremely effective at transporting seed from field to field. A few precautions leading up to harvest and during harvest can help manage any escaped problem weeds.

Since early August, farmers and consultants have been reporting what they believed were potassium (K) deficiency symptoms in soybean leaves located in the middle or upper canopy. This is not surprising in fields or portions of fields with soil-test values in the very low or low K interpretation categories. Moreover, K deficiency symptoms could develop at these growth stages with drought conditions, even in fields with adequate soil-test K levels.

Iowa State University Extension and Outreach field agronomists have reported the appearance of frogeye leaf spot in soybeans as much of the crop across Iowa enters the R3-R5 growth stage.

Soybean aphid is the most important insect pest of soybean in Iowa. Foliar insecticides, mostly pyrethroids and organophosphates, have been the primary management tactic for soybean aphid in Iowa since 2001. Regular scouting and timely treatments will protect yield. Our research and extension program at Iowa State University (ISU) is focused on evaluating insecticide efficacy for soybean aphid on a wide range of products.